Wide range amplifier circuits



Jan. 13, 1942. Q sc 2,269,693

WIDE RANGE AMPLIFIER CIRCUITS Filed Oct. 26; 1939 T COMPEN$ATION ERESONANCE FREQUENCY\ l 0 FREQUENCY 1 if); 2 f

g 3 /3 T0 our ur 1 l4 NETWORK 5 fl ll' rr' /6 *"J:,*" J:' 2 I 5 (Ti :1;2 ,5 705001205 0F :9 VIDEO vozmae '5 I fi= 1 =12 INVENTOR. 0770 ll.sol/ADE ATTORNEY.

Patented Jan. 13, 1942 Otto H. Schade, West Caldwell,

Radio Corporation of America,

Delaware N. J assignor to a corporation of Application October 26, 1939,Serial No. 301,319 2 Claims. (Cl. 179-171) My present invention relateto wide range amplifiers, and more particularly to video amplifiershaving a substantially flat'response over the operating frequency range.

One of the main objects of my present invention is to provide selectiveinverse feedback in a video amplifier thereby to secure completecompensation of deviations in gain or phase within a certain frequencyrange and without loss of normal gain.

Another important object of thisinvention is completely to compensatefor response-varying effects caused by circuit elements in the platecircuit of a wide band amplifier; particularly designed circuit elementsbeing inserted in the cathode circuit to effect the compensation.

Another object of my invention is to provide a video amplifier having aplate circuit constructed to impart a peak in the frequency-responsecharacteristic thereof, and the cathode circuit including a degenerativenetwork whose constants are chosen to compensate for said peak.

Still other objects of my invention are to improve the responsecharacteristic of video amplifiers, and more especially to providecompensation networks of the type disclosed hereinafter which arereadily and economically manufactured and assembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawing- Fig. 1 schematically illustrate the general circuitembodying the invention,

Fig. 2 shows a response characteristic of an amplifier employing theinvention,

Fig. 3 shows a video amplifier network utilizing the invention.

Referring now to the accompanying drawing, the amplifier tube l of Fig.1 has an input circuit connected to its cathode 2 and signal grid 3. Theoutput circuit is connected between the plate 4 and the cathode, andincludes the direct current source 5. The input voltage E1 is assumedapplied between the input electrodes. The plate load impedance isconsidered as including a portion Zn, or desirable fraction thereof, andthe portion Zx causing undesirable effects. The output voltage E0 may beconsidered as comprising the voltage En developed across n, and Exdeveloped across Zx. It can be shown that an impedance Zr, connectedbetween cathode and ground, can be designed to compensate for the avideo amplifier so that a age. The cathode undesirable effects causedbyZx in the plate circuit. The input voltage E1 is shown composed of ridto cathode voltage Eg and the voltage Ex developed across Zk.

In general, the ratio and phase relation of El to E remain unchangedwhen E1; and Ex furnish similar vector diagrams; in other words, thephase angle between Eg and Ek is equal to the phase angle between En andEx at any one frequency. Further, if the following relation holds true:

the effects of an impedance Zk can be eliminated by inserting Zx in theplate circuit (as disclosed in my application Serial No. 301,320, filedOctober 26, 1939) and, conversely, the effects of an undesirablefraction Zx of the total plate impedance can be eliminated by insertingZ1; in the cathode circuit, Compensation over a frequency range requiresthat Equation 1 holds true over that range. If, therefore, the undesiredeffect in the plate circuit is aresonance effect, Zx is found to be theimpedance'of a certain resonant circuit. Z1; is then designed as asimilar resonant circuit having an equal resonance rise and an impedanceat resonance which eventually resolves itself to:

I 1 a:(r) (2) ZMT) gm(lc) n Consider, now, a frequency-responsecharacteristic as shown in Fig. 2. Frequencies are plotted againstvoltage, or gain, as ordinates. The solid curve is the voltage outputwithout compensation, and rises to a peak near the cut-off frequency fc.It is desired to obtain the dotted curve; the shaded area must beeliminated. Assume that its shape is closely matched by the resonancecurve of a single parallel tuned circuit. Its resonant voltage is thenEm) as shown; the normal voltage desired being Eno). The impedance Zk(r)of the correction circuit has a positive value given by- Equation 2 andcan be realized. It is not desirable to design the coupling network ofdip occurs in the response characteristic, since in such case a negativevalue of Zk(r) would result and this would cause a loss of gain beforecompensation would be had.

In Fig. 3 there is shown a video amplifier network employing acorrection circuit in the oathode lead of the video amplifier tube 5.This tube may be a pentode of the 1851 type and has its signal gridliconnected to a source of video voltis connected to ground throughresistor I which is shunted by condenser 8, and the latter condenser isin turn shunted by a coil 9. The plate ll of tube 5 is connected to thepositive terminal of the direct current source I2 through a path whichincludes coil [3, coil [4 and resistor l5 arranged in series. Thenegative terminal of current source l2 may be established at groundpotential, and screen grid I6 will be connected to a desired positivepotential point on current source l2. The suppressor grid is connectedto a point on the current source which is at a substantially lowerpotential than the screen grid connection point. The junction of coilsl3 and I4 is connected to the signal grid of a following video amplifiertube 20, the usual coupling condenser 2| and grid leak resistor 22 beingincluded in the coupling network to tube 20.

The coupling network between tubes 5 and 20 carries of one pi sectionand one-half section. Coil M has a value which is one-half the value ofcoil I3; the symbol Cp denotes the shunt plate capacity shown in dottedlines, and the symbol Cg denotes the shunt capacity due to the gridcircuit of the following tube and is also shown in dotted lines. Theshunt capacities C1 and C2, both shown dotted, are the respectivedistributed capacities of coils l3 and M.

The cut-off frequency of a low pass filter may be expressed as follows:

In the much used single half-section filter (peaking coil) the value Cin Equation 3 is: 0:2 (C -l-C'g), while for the circuit shown in Fig. 3,C is equal to C The type 1851 tube has a capacity relation Cg equals twotimes Cp, which makes possible the use of the low pass filter shown inFig. 3 for which this relation is required. The cut-oil frequency fc forFig. 3 is, hence, theoretically three times higher for the same gain(same value of resistance of resistor !5) than for the normal circuitwith a single peaking coil. Actually, the coil capacities C1 and 02cause about 15% reduction of this ratio. This follows from the fact thatthe following relation exists:

Normally, the useful frequency range of this circuit for video purposesis approximately 0.7 ,fc, because it has a characteristic similar tothat shown by the solid curve in Fig. 2. The peak near cut-off isundesirable, and is eliminated by a correction impedance network l-8-9in the cathode circuit of tube Saccording to the discussion inconnection with Fig. 1. Actual testing of an amplifier having sevencircuits in cascade, and each circuit being of the type shown in Fig. 3,showed that one compensation network for the cut-off peak inserted inthe cathode circuit of one of the tubes was sufficient for equalizationof the entire video amplifier. The computed cutoff frequency of theamplifier was found to be 0:9 megacycles on 4 stages. Tests showed theend of the useful range to occur at 8 megacycles without causingtransients. cuit thus extended the useful range to 0.89 fc.

The present invention is not limited to the specific circuit shown inFig. 3, but may be used to correct other response characteristics ortransients originating in any part of an amplifier circuit by insertinga selective degenerative net- The equalizing cirill work in the cathodecircuit, or by inserting gain increasing circuit elements in the platecircuit according to the discussion in connection with Fig. 1. Becauseof the dependence of the correction impedance on gm, a voltageadjustment for the grid bias or screen-voltage of tube 5 should beprovided for the correcting stage which permits one to vary gm.Re-adjustment of this control is necessary only to correct changes of gmduring the life of the tubes, the correct setting being apparent by theabsence of transients in the amplified picture signal.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. A wide range video amplifier capable of amplifying frequencies up toapproximately 9 megacycles comprising an electron discharge tube of thetype including input and output electrodes, a video input circuitconnected to the input electrodes, a load circuit including resistiveand reactive components connected between the output electrodes. saidreactive component tending to produce an undesirable deviation in thefrequency response characteristic at the high frequency end of therange, and a reactive network in the space current path of said tube,common to the input and output circuits of the tube, for compensatingfor said undesirable deviation, said reactive network comprising theshunt connection of a resistance, a capacitance and an inductancebetween the tube cathode and ground and being resonant to the highfrequency end of the video range and having its impedance matched tothat of the reactive component of the output load circuit at said end ofthe range, and said load circuit comprising a low-pass filter and aresistance in series.

2. A wide range video amplifier capable of amplifying frequencies up toapproximately 9 megacycles comprising an electron discharge tube of thetype including input and output electrodes, a video input circuitconnected to the input electrodes, 2. load circuit including resistiveand reactive components connected between the output electrodes, saidreactive component tending to produce an undesirable deviation in thefrequency response characteristic at the high frequency end of therange, a reactive network in the space current path of said tube, commonto the input and output circuits of the tube, for compensating for saidundesirable deviation, said reactive network comprising the shuntconnection of a resistance, a capacitance and an inductance between thetube cathode and ground and being resonant to the high frequency end ofthe video range and having its impedance matched to that of the reactivecomponent of the output load circuit at said end of the range, said loadcircuit comprising a pair of coils and a resistance in series, and asecond amplifier tube having its input electrode connected to the commonterminal between the pair of coils.

OTTO H. SCHADE.

